This invention pertains generally to the preparation of silicon carbide foams and particularly to a method of preparing silicon carbide foams, having controlled structure and properties, from organosilicon polymers.
Because of its unique chemical and physical properties; extreme chemical inertness, the ability to withstand very high temperatures (&gt;2000 C), very high hardness, low coefficient of thermal expansion, excellent thermal conductivity and semiconducting properties, silicon carbide (SiC) has found an important place as an industrial material. While it is these unique properties that make SiC such an attractive material for many applications, they also are drawbacks, particularly insofar as the ability to produce or employ this material in different forms.
Silicon carbide can be made by heating silicon dioxide and carbon or graphite to temperatures in excess of 1500 C. The SiC whiskers formed by this process are known to possess a very high strength to weight ratio and oxidative stability. It would be desirable to be able to employ these whiskers directly as a reinforcing material in ceramic or metal composites. However, because of the various mechanical processes required to incorporate these whiskers into composites they become severely damaged and lose most of the mechanical properties that make them desirable in the first place. This same process can be used to produce to produce monolithic SiC structures. However, it is very difficult to get uniform reaction.
In those instances where only a high temperature coating is required, a chemical vapor deposition method can be used to produce SiC. Generally, SiC is deposited by heating a mixture of gases, for example, dichloromethylsilane and hydrogen, to a temperature above 1200 C; although, as discussed in U.S. Pat. Nos. 4,159,259 and 4,513,030 other organosilicon compounds can be used. However, unless the SiC coating is very thin it is difficult to get it to adhere well to the substrate, particularly where there is a significant difference in the coefficient of thermal expansion between the substrate and the SiC coating.
Silicon carbide composites can also be used to fabricate porous structural materials. In such instance, organosilicon precursor materials are heated to temperatures above the appropriate glass transition temperature (Tg), the temperature at which the organosilicon precursor fuses or softens, to form a spinning melt and the molten material is spun into a fiber which is then pyrolized to form a SiC fiber or thread. This fiber or thread is subsequently spun into a fabric. Conventional lay-up methods are used to fabricate the structural material desired. Restricted fiber preform connectivity increases the time and cost of matrix deposition. The fact that the fiber fabrication and lay-up processes are complex causes this process to have extremely high processing costs.
Silicon carbide foams possess many desirable features, in particular high strength to weight ratios, consequently, they find wide utility as catalyst supports, high temperature filtering media, materials of construction and heat exchangers, among other things. However, these foams are particularly difficult to produce. For these reasons, significant effort has been put forth to fabricate them. As disclosed in U.S. Pat. No. 5,154,970, a open porosity substrate can serve as a skeleton to define the geometry of the structure. A SiC or other refractory coating is then chemical vapor deposited thereon. The substrate can then be burned or leached out, if desired, thereby forming a reticulated SiC structure. U.S. Pat. Nos. 3,946,039, 4,664,858 and 5,248,462 disclose investment casting type processes, and variations thereon, for preparing reticulated SiC or other refractory structures. However, none of these methods can provide the ability to tailor the internal structure (porosity) of the SiC foam and, in particular, they cannot be used to produce meso-porous SiC foams which for the purposes of the instant invention is defined as having pores which are about 10-100 .mu.m in diameter. Infiltration methods as discussed, for example, in Mazdiyanski et al., J. American Ceramic Soc., 61, pp. 504-508, 1978, can be used for reducing porosity or pore volume but the process is very time consuming and it is difficult to achieve uniformity. All these methods of preparing SiC foams or porous materials suffer from the disadvantage that fabrication is difficult and/or time consuming and processing costs are high. What is desired therefore, is a method of preparing monolithic SiC foams that is simple, inexpensive and will produce foams that have controlled porosity and sufficient structural strength so that they are machineable and/or may be produced to near net shape.
The present invention provides a method of economically producing monolithic SiC foams, substantially free of large defects or voids, having controlled porosity and sufficient structural strength such that the SiC foams can be machined. The method disclosed herein further provides for producing near net shape SiC foam parts.